WaterHeaterSize Tip

Water heater size calculator

For a tank water heater, enter how many showers, baths, dishwasher and washer loads happen in the busiest hour of the day. WaterHeaterSize adds up the hot water with the US Department of Energy worksheet figures and gives the first-hour rating to look for on the label. The example is a family of four on a weekday morning: three showers, a shave, some dishes, a dishwasher and a washer load, 49 gallons.

Uses of hot water in the busiest hour of the day:

    Saved you some time? Leave a tip, entirely optional.

    For a tankless heater, switch tabs and enter the fixtures that may run at the same time and your incoming water temperature.

    How the water heater size calculator works

    Tank heaters are sized by their first-hour rating (FHR): how many gallons of hot water they deliver in one busy hour, starting with a full tank. WaterHeaterSize adds up your peak-hour demand with the US Department of Energy worksheet figures per use: shower 10 gal, bath 20, shaving 2, hand dishwashing or food prep 4, dishwasher 6 and clothes washer 7. Pick a heater whose FHR is at least that total. Tankless heaters are sized by flow and temperature rise instead: the flow of every fixture that may run at once (shower 2.0 GPM, bathroom faucet 1.0, kitchen faucet 1.5, tub 4.0, dishwasher 1.0, washer 2.0) times the rise from the incoming water to the setting. Heat needed = GPM × 500 × rise in °F, in BTU per hour; divide by efficiency for gas input, or by 3,412 for electric kW.

    Tankless size by flow at a 70°F rise (50°F water heated to 120°F)

    FlowHeat outputGas input at 95%ElectricAmps at 240 V
    1 GPM35,000 BTU/h36,842 BTU/h10.3 kW43 A
    2 GPM70,000 BTU/h73,684 BTU/h20.5 kW85 A
    3 GPM105,000 BTU/h110,526 BTU/h30.8 kW128 A
    4 GPM140,000 BTU/h147,368 BTU/h41 kW171 A
    5 GPM175,000 BTU/h184,211 BTU/h51.3 kW214 A
    6 GPM210,000 BTU/h221,053 BTU/h61.5 kW256 A

    Groundwater is colder in northern climates in winter (often 40°F or less), which raises the rise and the size you need. Low-flow fixtures reduce it. Gas units need correctly sized gas lines and venting, and electric units need a dedicated circuit; have a licensed installer confirm the final choice. Results are estimates for planning.

    Frequently asked questions

    What size water heater do I need?

    For a tank, one with a first-hour rating at least equal to your peak-hour demand. For tankless, one that supplies your simultaneous flow at your temperature rise.

    What is a first-hour rating?

    The gallons of hot water a tank heater can supply in one hour starting full. It is on the EnergyGuide label and depends on tank size and burner or element power.

    How much hot water does a shower use?

    About 10 gallons of hot water for a typical shower with a 2 GPM head, by the DOE worksheet figure.

    Is WaterHeaterSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

    More tools